Prognosis of Ph-positive acute lymphoblastic leukemia (ALL) remains poor because of frequent treatment failure and rapid relapse. ALL cells are significantly protected against drug treatment by stromal cells in their microenvironment, but the molecular signals that mediate the protection are not well-defined. All cells, and in particula cells of the immune system, are covered by a dense layer of carbohydrate-modified proteins and glycolipids, of which the function has not been explored. However, the first step in the communication between ALL cells and the stroma takes place through cell surface structures, of which carbohydrates are likely to play key roles. Our preliminary data show that the polyLacNAC-binding lectin Galectin-3 (Gal3) is part of the microenvironment created by stromal cells and identify extracellular Gal3 as a communication molecule between ALL cells and the stroma that protects ALL cells against drug treatment. We hypothesize that Gal3 in the ALL microenvironment is a molecular communicator that promotes Ph-positive ALL cell survival through extracellular lattice formation of glycosylated cell surface proteins and intracellularly b interactions with Bcr/Abl. Corollaries of this hypothesis are: Removal of Galectin-3 will 1) interrupt an important signal between these cell types and 2) will generate ALL cells that are more sensitive to drug treatment. The following Aims, with independent experiments that will be studied concurrently, will test these predictions: Aim 1 will determine the extracellular component of the signal by identification of the cell surface ligands/binding partners of Gal3 on Ph-positive ALL cells using different complementary approaches. Aim 2 will investigate the intracellular component of the signal by examining molecular interactions between Bcr/Abl and Gal3 and their effects on ALL cell survival. Aim 3 will determine the significance of extracellular and intracellular Gal3 to pre-B ALL cell function both in vitro, by comparing gene expression in gal3-/- and gal3+/+ pre-B ALL cells, in these cells stimulated with exogenous Gal3, and treated with nilotinib and in vivo through transplant of gal3-/- and gal3+/+ pre-B ALL cells into gal3 -/- and +/+ recipients and comparing homing, proliferation and the effect of vincristine and nilotinib treatment. These experiments together will be the first to fully characterize the significance of Gal3 in any type of cancer, and specifically will tease apart the function of this carbohydrate-binding protein in its extracellular location as communication messenger between stroma and ALL cells and in its intracellular location as an anti-apoptotic molecule.